1. Field of the Invention
The present invention relates to a buzzer driving device for driving a buzzer for a predetermined time after a switch is turned on.
2. Description of the Prior Art
Prior art buzzer driving devices for driving a warning buzzer when a seat belt of an automotive vehicle is not worn are constructed, for example, as shown in FIGS. 4 and 5.
As shown in FIG. 4, a series circuit consisting of a first resistor 3a used for the charging and discharging and a capacitor 3b are connected via an ignition switch 2 with a battery 1 of the automotive vehicle, which is a direct-current source, thereby forming a charging/discharging circuit 3. An emitter of a PNP transistor 5 is connected with the battery 1 via the ignition switch 2, a base thereof is connected with a common contact of the first resistor 3a and the capacitor 3b via a second resistor 6 for applying a bias to the base, and a collector thereof is grounded via a third resistor 7.
A diode 8 is connected in the reverse direction in parallel with the first resistor 3a. A fourth resistor 9 used for the discharging is connected between a cathode of the diode 8 and an earth. The collector of the transistor 5 is also connected with an input terminal of an inverter 10. One input terminal of a NOR gate 11 is connected with an output terminal of the inverter 10, and the other input terminal thereof is grounded via a buckle switch 12 which is turned on when the seat belt is not worn. An output terminal of the NOR gate 11 is connected with an input terminal of an oscillator 13 which forms a driving circuit 14 together with the inverter 10 and the NOR gate 11. A warning buzzer 15 is driven in response to an oscillatory output from the oscillator 14. Identified by 16 is a fifth resistor.
In another prior art shown in FIG. 5, an inverter 17 is provided in place of the transistor 5 and the third resistor 7 of FIG. 4. A source terminal of the inverter 17 is connected with a battery 1 via an ignition switch 2, an input terminal thereof is connected via a second resistor 6 with a contact of a first resistor 3a and a capacitor 3b, and an output terminal thereof is connected with an input terminal of the inverter 10.
In the construction of FIG. 4, when the ignition switch 2 is turned on while the buckle switch 12 is on because the seat belt is not worn, the voltage at point A at one end of the first resistor 3a, i.e., at its end leading to the ignition switch 2, rises to a source voltage V.sub.BAT of the battery 1 as shown in FIG. 6(a). At this stage, the transistor 5 is turned on at the same time when the ignition switch 2 is turned on, with the result that the voltage at point C where the collector of the transistor 5 is located rises as shown in FIG. 6(c).
Since the capacitor 3b is charged by a current from the battery 1 via the first resistor 3a, the voltage at point B at the other end of the first resistor 3a, i.e. at its end leading to the capacitor 3b rises at a time constant (=CR) determined by a resistance value (=R) of the first resistor 3a and a capacity (=C) of the capacitor 3b as shown in FIG. 6(b). The transistor 5 is turned off when the voltage at point B rises to or above a threshold value V.sub.t, and the voltage at point C falls as shown in FIG. 6(c). Thus, the NOR gate 11 is in its operative state only for a period T (see FIG. 6(c)) during which the transistor 5 is on. While the NOR gate 11 is in its operative state, the buzzer 15 is driven by the oscillator 13. The buzzer 15 makes buzzing sounds during the predetermined period (period T) immediately after the start of the engine following the turning-on of the ignition switch 2, thereby notifying passenger(s) of the automotive vehicle that seat belt(s) is/are not worn.
The construction of FIG. 5 operates similarly to the above construction. Specifically, the voltages at points A and B, i.e., at the opposite ends of the first resistor 3a and the voltage at point C' which corresponds to point C in FIG. 4 and is an output terminal of the inverter 10 vary as shown in FIGS. 6(a), 6(b) and 6(c), respectively.
However, with the constructions of FIGS. 4 and 5, the buzzer 15 may mistakenly operate in the following case because the threshold value (V.sub.t) of the transistor 5 or the inverter 17 is constant. When the seat belt is left unworn even after the warning by the buzzer 15, the source voltage may temporarily fall for a certain reason, thereby causing the voltage at point B to fall to or below the threshold value as shown in FIG. 6(b). When the source voltage returns to its previous level thereafter, the buzzer 15 is driven for the predetermined period T in the same way as when the ignition switch 2 is turned on, with the result that the buzzer operates when it should not.
An object of the invention is to avoid an erroneous operation of the buzzer.